In recent years, with the frequent haze events in China, PM2.5 the originally obscure jargon began into the folk, has become a hot topic in the streets of heat. But for the initial contact with the people in the term, "PM2.5" and the associated with a series of terms of PM10, TSP "may make people feel somewhat foggy and incomprehensible. In fact, noun of countrywide science and technology appraisal committee has in February 2013 to PM2.5 a easy to understand Chinese name -- fine particulate matter ", which" PM "is English contents matter (particulate matter) acronym, and" 2.5 "emphasis is particles suspended in the atmosphere aerodynamic equivalent diameter is less than or equal to 2.5 microns in that part. PM2.5 particle size is very small, probably less than the diameter of a human hair twenty one (Figure 1), which can be in the atmosphere retention for up to several days long, even transported to the far distance, thus affect atmospheric visibility and human health and climate change on regional and global scales.
It may have been noted that the definition of a PM2.5 appears to be a highly sophisticated definition of the word - "aerodynamic equivalent diameter". This is because the particles in the atmosphere are not spherical in rule, so it is difficult to define and measure the diameter of the particles directly. So in the actual operation, if the particles through instrument detection of aerodynamic characteristics (subsidence rate, retention time, the position of deposition, etc.) with diameter less than or equal to 2.5 microns and a density of 1 g / cm3 of the spherical particles are consistent, it is said to PM2.5.
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Figure 1 PM2.5 particle size |
Figure 2 aerodynamic diameter distribution of atmospheric particulate matter (J C Chow et al., 2012) |
Understand the concept of "aerodynamic equivalent diameter", it is not difficult to understand environmental air quality research in several other common particles, such as TSP, PM10 and PM1, ultrafine particles and nano particles, similar to their definitions and PM2.5, just corresponds to different particle size range, specific classification and the corresponding chemical group points is shown in Figure 2.
Mention of atmospheric particulate matter and PM2.5, have to introduce another term that is closely related to the aerosol. Aerosol (Aerosol) is a relatively stable suspension system (Wang Mingxing, 1999), which is a liquid or solid particle dispersed uniformly in a gas. Particulate matter in the atmosphere (solid state) is a part of the aerosol, usually refers to the particle size of 0.003-100 microns in diameter. Therefore, the relationship between PM2.5, atmospheric particulate matter and aerosols can be expressed in Figure 3.
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Fig 3 Schematic diagram of the relationship between PM2.5, atmospheric particulate matter and atmospheric aerosol |
Main source of PM2.5
The source of PM2.5 is very complex, mainly divided into two kinds of natural and human. Natural source is mainly by the volcanic eruption, the foam of the sea, sandstorm, ground dust and biomass burning (such as forest fires) and emissions from plants and other natural sources, anthropogenic source is mainly industrial and human life caused by the discharge of, including fossil fuel combustion, family firewood burning burning, factory emissions, road dust, construction dust and vehicle exhaust emissions and other (Figure 4).
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Fig. 4 the natural and anthropogenic sources of PM2.5 |
PM2.5 is a source of diversity, resulting in the complexity of its components. According to the formation of the way, PM2.5 components can be divided into one group and the two group. One group included directly in the form of particulate matter discharged group and in the condition of high temperature in the form of a gas discharge, in ambient air dilution and cooling to condense into particulate group points, such as dust, organic carbon, black carbon; second group sub is generally composed of gaseous air pollutants and other groups by were phase or heterogeneous phase chemical reaction formation, such as sulfate, nitrate, ammonium, secondary organic carbon. According to our laboratory in 2003 in fourteen cities nationwide analysis results show (Figure 5), of PM2.5 chemical composition, organic matter (organic matter OM), geological dust and sulfate accounted for larger proportion and developed areas of Europe and the United States are obviously different.
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Figure 14 the average chemical composition of PM2.5 in 5 cities in China (J J et al. Cao, 2012) |
Research significance of PM2.5
PM2.5 can directly by scattering and absorption of solar radiation affect the entire Earth atmosphere system radiation balance, can also be by acting as cloud condensation nuclei, change cloud microphysical and chemical properties and effects of climate change; at the same time, PM2.5 on visible light scattering and absorption can lead to surface visibility decreased the haze frequency was significantly increased, to the city landscape and people's daily life caused by the direct effect; in addition, due to the PM2.5 compared to coarse particles, the specific surface area is large, the environment in retention time is longer, the adsorption of harmful substances and heavy metal more and more easy to enter the body of bronchial and alveolar region. Therefore, on the human body harm. Therefore, we will continue to launch a series of articles on the PM2.5 related knowledge, monitoring tools, evaluation criteria and research trends and other aspects of a more comprehensive introduction, please look forward to!
Reference
1 Junji Cao et al., 2014, "PM2.5 and the environment", science press.
2 the United States Environmental Protection Agency website, http://www.epa.gov/air/particlepollution/health.html
3 J Chow C Cao J J et al. brief A history of PM2.5 its measurement and adverse effects. Journal of Earth Environment 5: 1019-1029..
4 Wang Mingxing, 1999 atmospheric chemistry, Beijing: China Meteorological Press, china.
5. Cao J J et al., 2012. Winter and summer PM2.5 chemical compositions in fourteen Chinese cities, Journal of the Air & Waste Management Association, 62(10): 1214-1226.
